Method for controlling fresh air injection into the exhaust of an internal-combustion engine, notably of a motor vehicle

ABSTRACT

The present invention relates to a method for controlling air injection into the exhaust of an internal-combustion engine running on a cycle during which the expansion phase (D) thereof has a greater stroke than the compression phase (C) thereof and comprising at least one cylinder ( 10 ) with a combustion chamber ( 18 ), a piston ( 16 ), at least one intake means ( 24 ) with a valve ( 28 ) associated with an intake pipe ( 26 ), at least one exhaust means ( 30 ) with a valve ( 34 ) associated with an exhaust pipe ( 32 ), fresh air injection means ( 48 ) at the exhaust and an exhaust line ( 40 ) comprising at least one exhaust gas depollution means ( 46 ). 
     According to the invention, the method consists, during the cold start phase of this engine and during the expansion phase (D) thereof, in injecting fresh air into the exhaust and in opening exhaust valve ( 34 ) when the pressure in the combustion chamber is lower than the pressure prevailing at the exhaust so as to feed into the exhaust gas present in the combustion chamber the fresh air coming from this exhaust.

FIELD OF THE INVENTION

The present invention relates to a method for injecting fresh air into the exhaust of an internal-combustion engine, notably for a motor vehicle.

It more particularly relates to such a method applied to an engine running on a cycle during which the expansion phase is longer than the compression phase, as for a Miller cycle.

BACKGROUND OF THE INVENTION

As it is generally known, the pollutants emitted during cold start of internal-combustion engines, notably spark-ignition engines, are a problem for car manufacturers.

In fact, during this cold start, the exhaust gas depollution means these engines are usually equipped with, such as a catalyst, do not have a sufficiently high operating temperature, referred to as light-off temperature, to be able to treat these pollutants efficiently.

A large part of the pollutants contained in this exhaust gas, such as unburnt hydrocarbons (HC) and/or carbon oxides (CO), is therefore discharged to the atmosphere without being treated beforehand, which represents a quite significant nuisance.

For this catalyst to be able to rapidly reach its operating temperature, it is well known to provide fast light-off thereof by injecting fresh air into the exhaust line and upstream from this catalyst.

More particularly, it is notably known to inject this fresh air downstream from the exhaust valves so as to provide the exhaust gas circulating in the exhaust pipe with oxygen in order to achieve combustion with the unburnt HC contained therein.

Such a post-combustion allows, on the one hand, to directly reduce partly the HC and the CO, and on the other hand to increase the temperature of the exhaust gas that will thereafter flow through the catalyst while increasing the temperature thereof.

It has been observed that, to achieve this post-combustion, it is necessary to feed the fresh air as close as possible to the exhaust valves, a place where the exhaust gas temperature is the highest. However, since the air supplied is cold in relation to the burnt gas that is very hot, the mixing rate is a determining factor for this post-combustion. In fact, if a large amount of cold air is present, the chemical reactions are stopped. On the other hand, if this amount of air is insufficient, the heat release due to the reaction of the mixture is not sufficient to start the combustion of this mixture.

This therefore requires very precise adjustment of the amounts of fresh air and of exhaust gas present at the exhaust in order to obtain the appropriate mixture, failing which it is not possible to achieve post-combustion of the fresh air/exhaust gas mixture.

The present invention aims to overcome the aforementioned drawbacks by means of a method for injecting fresh air into the exhaust which allows to achieve post-combustion of the fresh air/exhaust gas mixture whatever the quality of this mixture.

SUMMARY OF THE INVENTION

The invention therefore relates to a method for controlling air injection into the exhaust of an internal-combustion engine, notably for a motor vehicle, running on a cycle during which the expansion phase thereof has a greater stroke than the compression phase thereof and comprising at least one cylinder with a combustion chamber, a piston sliding in a reciprocating motion between a top dead center and a bottom dead center, at least one intake means with a valve associated with an intake pipe, at least one exhaust means with a valve associated with an exhaust pipe, fresh air injection means at the exhaust and an exhaust line comprising at least one exhaust gas depollution means, characterized in that it consists, during the cold start phase of this engine and during the expansion phase thereof, in injecting fresh air into the exhaust and in opening the exhaust valve when the pressure in the combustion chamber is lower than the pressure prevailing at the exhaust so as to feed into the exhaust gas present in the combustion chamber the fresh air coming from this exhaust,

The method can consist in opening the exhaust valve at a crank angle for which the pressure in the combustion chamber is lower than the pressure prevailing at the exhaust.

The method can consist in injecting fresh air into the exhaust pipe.

The method can consist in injecting fresh air into the exhaust manifold of the exhaust line.

The method can consist in injecting fresh air into the exhaust before opening of the exhaust valve.

The method can consist in injecting fresh air into the exhaust after opening of the exhaust valve.

The method can consist in carrying out a succession of exhaust valve opening and closing cycles in order to feed into the combustion chamber the fresh air coming from the exhaust.

BRIEF DESCRIPTION OF THE FIGURES

Other features and advantages of the invention will be clear from reading the description hereafter, given by way of non-limitative example, with reference to the accompanying figures wherein:

FIGS. 1 to 4 are diagrams showing an internal-combustion engine in different running configurations and using the method according to the invention, and

FIG. 5 shows the various engine valve lift laws according to FIGS. 1 to 4, between an open position (O) and a closed position (F) as a function of crank angle (V).

DETAILED DESCRIPTION

In FIGS. 1 to 4, the engine shown is an indirect-injection internal-combustion engine, notably a gasoline and preferably a spark-ignition engine. This example is not limitative and the invention described hereafter can also apply to a direct-injection internal-combustion engine, preferably an auto-ignition engine, in particular of diesel type.

The engine described has the specific feature of running on a specific cycle, such as the Miller cycle, during which its expansion phase has a longer stroke than its compression phase, as described in the rest of the description.

This engine comprises at least one cylinder 10 with a cylinder body 12 closed by a cylinder head 14. A piston 16 slides within cylinder body 12 in a rectilinear reciprocating motion under the effect of a rod controlled by a crankshaft (not shown), thus forming a combustion chamber 18. This chamber is thus delimited by the cylinder head, lateral wall 20 of the cylinder body and upper part 22 of the piston.

This piston moves between a top dead center position (PMH in the figure) where upper part 22 of piston 16 is the closest to cylinder head 14 and a bottom dead center position (PMB) where this upper part is the furthest from this cylinder head.

The cylinder head carries at least one intake means 24 with an intake pipe 26 from which access to combustion chamber 18 is controlled by an intake valve 18. The cylinder head also carries at least one exhaust means 30 with a exhaust pipe 32 associated with an exhaust valve 34 for communication with this chamber.

Opening/closing of the intake and exhaust valves is controlled by specific means. The latter must allow the lift laws of these valves to be varied, as regards their opening/closing time as well as their opening height, independently of one another or in combination with one another. By way of example, these means are of camshaft type, more commonly known as VVT (Variable Valve Timing), VVL (Variable Valve Lift) or VVA (Variable Valve Actuation).

In the example described in connection with FIGS. 1 to 4, intake valve 28 and exhaust valve 34 are provided with VVA type means, 36 and 38 respectively, allowing the lift laws thereof to be varied.

The engine described is an indirect-injection engine with fuel injection means (not shown) carried by the cylinder head that spray fuel into intake pipe 26 so as to obtain a fuel mixture with the intake air circulating therein. This fuel mixture is then fed into combustion chamber 18 through the intake valve.

This type of fuel injection can be advantageously associated with means for igniting the fuel mixture present in chamber 18, such as spark plug ignition (not shown).

Exhaust means 30 are connected to an exhaust line 40 that essentially comprises an exhaust manifold 42 connected to the outlet of exhaust pipe 32, an exhaust tube 44 and depollution means for the gas circulating in this line, such as a catalyst 46, advantageously a three-way catalyst.

This engine also comprises air injection means 48 at the exhaust. These means include an air injector (symbolized in the figures by dotted line 50) that is connected by any known means to an air pump 52. This injector is arranged on exhaust pipe 32 in such a way that the air jet coming from this injector reaches the inside of this pipe advantageously as close as possible to exhaust valve 34 and downstream thereof, considering the direction of circulation of the exhaust gas from the combustion chamber to exhaust manifold 42.

Means 36, 38 controlling valves 28, 34, the fuel injection means and air injection means 48 are controlled by a computing unit (not shown), more commonly referred to as engine calculator, an engine is usually provided with.

The purpose of this calculator is notably to control opening/closing of the valves, the fuel injection parameters, such as the injection time in the engine cycle and/or the fuel injection duration, and the injection parameters of the air injected into the exhaust pipe.

The description of the method hereafter is given in connection with a spark-ignition indirect-injection engine of FIGS. 1 to 4 associated with FIG. 5 that illustrates the lift laws of its intake and exhaust valves.

In the example of FIG. 1, the engine is in the intake phase A configuration, with a stroke Ca of piston 16 from top dead center PMH to bottom dead center PMB.

In the vicinity of top dead center PMH, control means 36 therefore control the opening of intake valve 28 until the piston reaches its bottom dead center PMB in order to feed into combustion chamber 18 the fuel mixture present in the intake pipe. Control means 38 control the maintenance in closed position of exhaust valve 34 throughout this intake phase.

From this bottom dead center PMB, the piston has a reverse stroke when achieving a phase referred to as compression phase C where it moves from its bottom dead center PMB to its top dead center PMH. This piston displacement is divided in two parts, a discharge stroke Cr and a compression stroke Cp, as described in detail hereafter.

As shown in FIGS. 2 to 5, during discharge stroke Cr, intake valve 28 is open and piston 16 performs stroke Cr between the bottom dead center PMB and a crank angle Va where intake valve 28 is moved toward the closed position thereof. During this stroke, part of the fuel mixture present in chamber 18 leaves this chamber through pipe 26.

Of course, it is within the capacity of the person skilled in the art to determine the crank angle Va allowing to meet the engine power request.

Furthermore, this person skilled in the art will make sure to close the intake valve so that the fuel mixture present in the intake pipe is not discharged out of the engine and remains confined in this intake pipe (or, at worst, in the intake manifold this engine usually comprises).

From crank angle Va, the intake and exhaust valves are closed and piston 16 continues to move until it reaches top dead center PMH so as to achieve compression stroke Cp of the fuel mixture between angle Va and this PMH referred to as compression top dead center.

From this compression top dead center PMH, the fuel mixture present in the combustion chamber is ignited by generating burnt gas (or exhaust gas). After this combustion, the engine achieves an expansion phase D between this PMH and PMB, with an expansion stroke Cd and an air injection stroke Ci, as illustrated in FIG. 3.

During this expansion phase and in cases where the engine is in a cold start state, notably when catalyst 46 has not reached its light-off temperature and the temperature of the exhaust gas is not high enough to provide fast catalyst heating, fresh air is injected into the exhaust so as to increase the exhaust gas temperature.

More precisely, fresh air is predominantly injected inside the combustion chamber because the hottest areas, locally rich (fuel desorbed in cracks, etc.) and at high pressure are within this chamber.

Therefore, as illustrated in FIGS. 3 and 5, during expansion phase D, the piston follows an expansion stroke Cd during which this piston moves from top dead center PMH to a crank angle Ve before bottom dead center PMB.

Control means 38 control then opening of exhaust valve 34 at this angle. This angle Ve is preferably determined when the pressure in the combustion chamber is lower than the pressure prevailing at the engine exhaust, generally considered at the level of exhaust manifold 42 or of the exhaust pipe.

These various pressures can be evaluated from charts contained in the calculator and established in connection with the different operating points of the engine. These pressures can also be measured, for example by pressure detectors arranged in the combustion chamber and in the exhaust manifold/exhaust pipe.

Simultaneously or quasi-simultaneously with this exhaust valve opening, air pump 52 is actuated and fresh air is fed through injector 50 into exhaust pipe 32 and in the vicinity of this valve. Of course, exhaust gas is already present in this pipe and this thus provides a mixture of fresh air and of exhaust gas.

As already mentioned, it is in the combustion chamber that the conditions are the most favourable for combustion, as a result of the high temperatures prevailing therein and of the presence of unburnt hydrocarbons in the cracks of this chamber.

Thus, upon opening of exhaust valve 34 and during stroke Ci from crank angle Ve to the vicinity of PMB, fresh air is sucked into the chamber through the open exhaust valve. This allows a reactive mixture to be obtained more readily in the combustion chamber with the residual burnt gas and the fresh air fed into this chamber.

Post-combustion of the residual burnt gas is thus initiated in the chamber with heat release until the engine has completed its expansion phase at the top dead center referred to as expansion PMB, with closing of the intake valve.

Alternatively, it is possible to inject fresh air into the exhaust manifold which can only simplify the air injection means.

In this context, injecting air into the exhaust has been mentioned above, which comprises feeding this air into the exhaust pipe or into the exhaust manifold.

As it is well known, the operating mode of the engine continues with an exhaust phase E and a stroke Ce of the piston between the expansion PMB and its PMH, as illustrated in FIGS. 4 and 5.

During this phase, the high-temperature gas contained in combustion chamber 18 is expelled from this chamber towards exhaust pipe 32 through exhaust valve 34 which is in open position, and under the impulse of the displacement of piston 16 towards top dead center PMH.

This gas is then sent to exhaust line 40 where it flows through catalyst 46 while speeding up its temperature rise through thermal exchange.

In the vicinity of the piston PMH, the exhaust valve is closed, the operating cycle of the engine is resumed from FIG. 1 and continued until catalyst 46 has reached its operating temperature.

The time required to obtain depollution of the exhaust gas circulating in line 40 is thus greatly shortened.

The present invention is not limited to the embodiment examples described above and it encompasses any variant and equivalent.

Notably, it is possible for air pump 52 to be in operation before the exhaust valve of FIG. 3 opens. This affords the advantage of allowing suction of fresh air that has already been placed in the vicinity of the exhaust valve, when the latter is open.

It is also possible to perform, from angle Ve, a succession of exhaust valve opening/closing cycles associated with air injection until the piston has reached the PMB position. This has the effect of promoting air and burnt gas mixing in the combustion chamber. 

1) A method for controlling air injection into the exhaust of an internal-combustion engine, notably for a motor vehicle, running on a cycle during which the expansion phase thereof has a greater stroke than the compression phase thereof and comprising at least one cylinder with a combustion chamber, a piston sliding in a reciprocating motion between a top dead center and a bottom dead center, at least one intake means with a valve associated with an intake pipe, at least one exhaust means with a valve associated with an exhaust pipe, fresh air injection means at the exhaust and an exhaust line comprising at least one exhaust gas depollution means, characterized in that it consists, during the cold start phase of this engine and during the expansion phase thereof, in injecting fresh air into the exhaust and in opening exhaust valve when the pressure in the combustion chamber is lower than the pressure prevailing at the exhaust so as to feed into the exhaust gas present in the combustion chamber the fresh air coming from this exhaust. 2) A control method as claimed in claim 1, characterized in that it consists in opening the exhaust valve at a crank angle for which the pressure in the combustion chamber is lower than the pressure prevailing at the exhaust. 3) A control method as claimed in claim 1, characterized in that it consists in injecting fresh air into exhaust pipe. 4) A control method as claimed in claim 1, characterized in that it consists in injecting fresh air into exhaust manifold of exhaust line. 5) A control method as claimed in claim 1, characterized in that it consists in injecting fresh air into the exhaust before opening of exhaust valve. 6) A control method as claimed in claim 1, characterized in that it consists in injecting fresh air into the exhaust after opening of exhaust valve. 7) A control method as claimed in claim 1, characterized in that it consists in carrying out a succession of exhaust valve opening and closing cycles in order to feed into combustion chamber the fresh air coming from the exhaust. 